A fuel cell stack is an electric power generation system which oxidizes a fuel by an electrochemical process to directly convert energy discharged owing to an oxidizing reaction into electric energy. The fuel cell stack comprises a membrane electrode assembly in which both side surfaces of a polymer electrolyte membrane for selectively transporting hydrogen ions are held by a pair of electrodes made of a porous material. Each of the pair of electrodes contains, as a main component, carbon powder which carries a platinum based metal catalyst, and comprises a catalyst layer which comes in contact with the polymer electrolyte membrane, and a gas diffusion layer formed on the surface of the catalyst layer and having both air permeability and electron conductivity.
In a fuel cell vehicle in which a fuel cell system is mounted as a power source, an electric power is generated by the fuel cell stack in a high output region having a satisfactory electric power generation efficiency, and the power is supplied to a traction motor from both the fuel cell stack and a secondary battery or only from the fuel cell stack. On the other hand, in a low output region having a poor electric power generation efficiency, operation control is performed so that the electric power generation of the fuel cell stack is temporarily halted to supply the power to the traction motor only from the secondary battery. This temporary halting of the operation of the fuel cell stack in a low load region having a low electric power generation efficiency of the fuel cell system is referred to as an intermittent operation. In the low load region where the electric power generation efficiency of the fuel cell system lowers, the intermittent operation can be performed to operate the fuel cell stack in a region having a high energy conversion efficiency, which can increase the efficiency of the whole fuel cell system.
In Japanese Patent Application Laid-Open No. 2007-109569, a fuel cell system which performs an intermittent operation is disclosed. In the fuel cell system disclosed in the publication, in a case where the cell voltage of a fuel cell stack which has shifted to an electric power generation halt state becomes lower than a predetermined value, an air compressor is driven again to replenish an oxygen gas to the fuel cell stack, and the shortage of oxygen in a cathode of the fuel cell stack is eliminated to recover the cell voltage, thereby preventing the delay of a response to a request for electric power generation. At this time, the cell voltage excessively rises owing to the excessive supply of the oxygen gas, and the deterioration of the fuel cell stack is promoted sometimes. This is because the platinum catalyst included in the catalyst layer of the membrane electrode assembly is ionized and eluted in a high potential environment. The above publication also discloses a method (hereinafter referred to as high potential avoidance control) which controls the electric power generation so that the cell voltage does not exceed a predetermined upper limit voltage (hereinafter referred to as the high potential avoidance voltage), to avoid the above disadvantage.    Patent Document 1: Japanese Patent Application Laid-Open No. 2007-109569